The present invention relates generally to directive type communication systems and more specifically to phased array electronically steered antennas.
The common method of arranging a phased array antenna is to distribute microwave energy along a delay line in an IF scanner that has equally spaced outputs such that the phase of the outputs is known and fixed at each terminal. The difference in output phase .DELTA..phi. between each adjacent output terminal is a function of the output frequency, f.sub.o, and the angle the beam makes off broadside, .theta..sub.o, (i.e., the direction of the propagated beam from a perpendicular to the antenna array). The relationship is given by: .DELTA..phi. = 2.pi.s/c f.sub.o sin .theta..sub.o where s is the spacing between antenna elements in the array and c is the speed of light. More simply we can write: EQU .DELTA..phi. = k.sub.3 f.sub.o sin .theta..sub.o
Thus the output beam propagation direction .theta..sub.o is a function of the output frequency f.sub.o and the difference in output phase .DELTA..phi., which is linearly related to changes in output frequency for fixed delay line IF scanners.
To independently control the antenna propagation direction .theta..sub.o, prior systems have utilized phase shifters at each element of the array which are capable of introducing a progressive phase shift across the antenna array. These phase shifters can thus be controlled independently of the output frequency. However, the phase shift required for any desired beam direction is still dependent on the output frequency. Thus to change either output frequency or beam direction or both, the phase shift must be recalculated in each instance. This requires the use of expensive computing mechanisms especially where a rapid change in frequency and in some instances direction is required, e.g., radar tracking where the tracking frequency is rapidly changed to avoid detection and the use of countermeasures by the target.